1. Field of the Invention
The present invention relates to lasers and particularly to a resonantly pumped, erbium-doped, 2.8 micron solid state laser system with high slope efficiency at or near room temperature.
2. Description of the Prior Art
It is well known that the human body is comprised of approximately 70% water, with various human tissues containing about 60% to 90% of water, and bone and cartilage containing about 30% to 40% of water. Since the 2.8 micron wavelength has a substantially maximum absorption in water, this 2.8 micron wavelength is the ideal wavelength to use for a large variety of medical laser applications on the human body. The 2.8 micron wavelength also offers a controlled absorption or penetration depth of, for example, one micron in the human body. As a result, this 2.8 micron wavelength is extremely useful in surgical applications where very precise cuts in area and/or depth are needed while minimizing damage to good tissue, bone and/or cartilage adjacent to, or under, the area to be ablated. A 2.8 micron wavelength laser could be used for precise surgery in such exemplary applications as brain surgery, neurosurgery, ear surgery, eye surgery, plastic surgery, burn treatment, dentistry, and the removal of malignancies.
Current lasers for generating this 2.8 micron wavelength use a variety of host or lasant materials with various pumping techniques for exciting the lasant material. Typically these lasers are flashlamp pumped. Such flashlamp pumped lasers are large, inefficient and expensive.
The development of high power semiconductor lasers has led to renewed interest in resonant pumping of solid state lasers based on rare earth active ions. Most of this research has been confined to the use of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) semiconductor diode laser devices which perform at high power within the range of 750 to 870 nm (nanometers). These diode lasers have been used to pump Er.sup.3+ (erbium) at about 800 nm (as well as to pump Nd.sup.3+ at about 810 nm and Tm.sup.3+ at about 790 nm). One such diode laser pumped solid state laser is disclosed in U.S. Pat. No. 5,014,279, issued May 7, 1991 to Esterowitz et al. In this patent, an erbium-doped crystal laser is resonantly pumped by a pump beam at about 800 nm from an AlGaAs diode laser to enable the erbium-doped crystal laser to produce a laser emission at substantially 2.8 microns with about a 10% slope efficiency.
The 800 nm resonant pumping diagram for the 2.8 micron Er.sup.3+ 4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser is shown in FIG. 1. The .sup.4 I.sub.9/2 state is resonantly pumped by the 800 nm pump beam and the .sup.4 I.sub.11/2 upper laser state is populated as shown through the decay .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.11/2. The decay of the .sup.4 I.sub.9/2 state is primarily non-radiative. The radiative decay rate from the .sup.4 I.sub.9/2 state is more than two orders of magnitude lower than the non-radiative decay rate. Therefore, the radiative decay processes .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.13/2 and .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.15/2, which would bypass the upper laser state and therefore reduce the efficiency of the 2.8 micron laser, can be ignored. However, there is a power loss experienced in the 800 nm resonant pumping scheme shown in FIG. 1 due to the .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.11/2 phonon decay. This power loss reduces the slope efficiency in the 800 nm resonant pumping scheme of FIG. 1. More specifically, the maximum possible slope efficiency for a 2.8 micron laser pumped by the 800 nm resonant pumping scheme of FIG. 1 is given by .lambda..sub.pump /.lambda..sub.laser =28%.
Another loss mechanism that results in a still lower slope efficiency for the Er.sup.3+ 800 nm resonant pumping scheme is illustrated in FIG. 2. Essentially, FIG. 2 illustrates the Er.sup.3+ concentration-dependent self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2. The non-radiative self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 bypasses the .sup.4 I.sub.11/2 upper laser state and leads to a reduction in the pumping efficiency of the 800 nm-pumped 2.8 micron Er.sup.3+ laser. This self-quenching process is a phonon-assisted dipole-dipole interaction between nearby Er3+ ions in the crystal lattice. The probability for the occurrence of the self-quenching process increases at higher Er.sup.3+ concentrations due to the stronger dipole-dipole interaction for smaller separation between Er.sup.3+ ions. Therefore, the lifetime of the .sup.4 I.sub.9/2 state decreases at higher concentrations.
FIG. 3 illustrates the fluorescence decay from the .sup.4 I.sub.9/2 state for 4% and 30% concentrations of Er.sup.3+ in a YLF crystal host. The natural logarithm of the fluorescence intensity is shown plotted against time. The .sup.4 I.sub.9/2 state is excited by a pulsed dye laser having a pulse duration (.apprxeq.10 nano seconds) significantly shorter than the .sup.4 I.sub.9/2 lifetime. The .sup.4 I.sub.9/2 lifetimes are obtained from a linear least-squares fit (the solid line in FIG. 3) to the fluorescence data. Similar fluorescence decay data are obtained for two intermediate Er.sup.3+ concentrations, 8% and 16%, in the YLF crystal host. The .sup.4 I.sub.9/2 lifetimes for the 4%, 8%, 16% and 30% concentrations are given in the following TABLE 1.
TABLE 1 ______________________________________ PUMPING Er.sup.3+ CONCENTRATION .sup.4 I.sub.9/2 LIFETIME EFFICIENCY ______________________________________ 4% 6.46 .mu.s 100% 8% 6.39 .mu.s 99% 16% 6.10 .mu.s 94% 30% 5.02 .mu.s 78% ______________________________________
The lifetimes for the 4% and 8% samples are very nearly the same, implying that for the 4% Er.sup.3+ the .sup.4 I.sub.9/2 decay is due almost entirely to phonon decay to the .sup.4 I.sub.11/2 state. (The radiative decay rate from the .sup.4 I.sub.9/2 state is negligible compared to the phonon decay rate.) The quantum efficiency for populating the .sup.4 I.sub.11/2 upper laser state is given by .tau.(C)/.tau.(0), where .tau.(C) is the .sup.4 I.sub.9/2 lifetime for Er.sup.3+ concentration C, and .tau.(0) is the limiting value of the lifetime for an arbitrarily small Er.sup.3+ concentration. That is, .tau.(0) is due to purely phonon decay. Since .tau.(4%).apprxeq..tau.(8%), it can be assumed that .tau.(0).apprxeq..tau.(4%). Using this approximation, the quantum efficiencies .tau.(C)/.tau.(0) for pumping the .sup.4 I.sub.11/2 upper laser state are given in the above TABLE 1.
The maximum possible slope efficiency for the 800 nm-pumped 2.8 micron Er.sup.3+ :YLF laser with Er.sup.3+ concentration C is (.tau.(C)/.tau.(0))(.lambda..sub.pump /.lambda..sub.laser). From TABLE 1, the maximum possible slope efficiency for the 800 nm pumping scheme of FIG. 1 is therefore 22% for an Er.sup.3+ concentration of 30%.
TABLE 1 also shows that the reduced efficiency for the 800 nm-pumped 2.8 micron laser due to the self-quenching process (previously discussed in relation to FIG. 2) can be avoided by using a low Er.sup.3+ concentration. However, this approach is not suitable for the cw-pumped 2.8 micron Er.sup.3+ laser due to the importance of the upconversion process .sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.9/2 .sup.4 I.sub.15/2 for cw operation of the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 transition. This transition is nominally self-terminating due to the long lifetime (13.2 milliseconds or ms) of the .sup.4 I.sub.13/2 lower laser state relative to the lifetime (4.2 ms) of the .sup.4 I.sub.11/2 upper laser state. The upconversion process .sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.9/2 +.sup.4 I.sub.15/2, which is the inverse of the self-quenching process and is therefore increasingly efficient for higher Er.sup.3+ concentrations, effectively reduces the lower laser state lifetime and allows cw operation of the otherwise self-terminated 2.8 micron laser transition. This effect has been demonstrated for the cw-pumped 2.8 micron Er.sup.3+ :YLF laser, for which a slope efficiency of 0.7% was obtained for an 8% Er.sup.3+ concentration (See "CW and Pulsed 2.8 .mu.m Laser Emission from Diode-Pumped Er.sup.3+ :LiYF.sub.4 at Room Temperature" by G. J. Kintz, R. Allen, and L. Esterowitz, Appl. Phys. Letts., Vol. 50 (22), pp. 1553-1555 (June 1, 1987)), and a 10% slope efficiency was obtained for a 30% Er.sup.3+ concentration (See U.S. Pat. No. 5,014,279). This fundamental trade-off, i.e. higher cw efficiency due to the effective reduction in the lower laser state lifetime via the upconversion process for higher Er.sup.3+ concentration, and lower efficiency due to the self-quenching loss for higher concentration, can not be avoided in the 800 nm pumping scheme.
FIG. 4 illustrates the polarized absorption spectrum for 30% Er.sup.3+ :YLF in the 800 nm region. Since YLF is a uniaxial crystal, the absorption is shown for both the c-axis (the solid line) and the a-axis (the dotted line) polarizations. Note that the absorption spectrum in the 800 nm region is strongly polarized. The peak c-axis absorption is approximately five times stronger than the peak a-axis absorption. As a result of this weak a-axis absorption in the 800 nm region, a polarization-coupled beam-combining pumping scheme can not be employed in the 800 nm region. Also note the narrowness of the absorption spectrum for both polarizations in the 800 nm region. The strongest c-axis absorption peaks in the 800 nm region have a width of only 1 nm (FWHM or full width at half maximum).